Angiogenesis, which refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, is known to be a key element in tumor growth, survival and metastasis. Growth factors and their receptors, including epidermal growth factor (EGF), transforming growth factor-α (TGF-α), transforming growth factor-,δ (TGF-/3), acidic and basic fibroblast growth factor (aFGF and bFGF), platelet derived growth factor (PDGF), and vascular endothelial growth factor (VEGF), are thought to play a role in tumor angiogenesis. See Klagsbrun & D'Amore, Annual Rev. Physiol., 53: 217-239 (1991). Binding of these growth factors to their cell surface receptors induces receptor activation, which initiates and modifies signal transduction pathways and leads to cell proliferation and differentiation. VEGF, an endothelial cell-specific mitogen, is distinct among these factors in that it acts as an angiogenesis inducer by specifically promoting the proliferation of endothelial cells.
The biological response of VEGF is mediated through its high affinity receptors, which are selectively expressed on endothelial cells during embryogenesis (Millauer, Cell, 72: 835-846 (1993)) and during tumor formation. VEGF receptors (VEGFRs) typically are class III receptor-type tyrosine kinases characterized by having several, typically 5 or 7, immunoglobulin-like loops in their amino-terminal extracellular receptor ligand-binding domains (Kaipainen et ah, J. Exp. Med., 178:2077-2088 (1993)). The other two regions include a transmembrane region and a carboxy-terminal intracellular catalytic domain interrupted by an insertion of hydrophilic interldnase sequences of variable lengths, called the kinase insert domain (Terman et al., Oncogene, 6:1677-1683 (1991)). VEGFRs include>z,s-like tyrosine kinase receptor (flt-1), or VEGFR-I, sequenced by Shibuya et al., Oncogene, 5: 519-524 (1990), kinase insert domain-containing receptor/fetal liver kinase (KDR/fik-1), or VEGFR-2, described in WO 92/14248, filed Fe. 20, 1992, and Terman et al, Oncogene, 6: 1677-1683 (1991) and sequenced by Matthews et al, Proc. Natl. Acad. Sd. USA, 88: 9026-9030 (1991), although other receptors, such as neuropilin-1 and -2, can also bind VEGF. Another tyrosine kinase receptor, VEGFR-3 (flt-4), binds the VEGF homologues VEGF-C and VEGF-D and is more important in the development of lymphatic vessels.
The importance of VEGFR-I in regulation of pathological angiogenesis has been shown in in vivo experimental models. Deficiency of VEGFR-I tyrosine kinase domain results in decreased blood vessel formation in tumors, indicating a significant role of VEGFR-I tyrosine kinase in pathological angiogenesis (Hiratsuka et al., Cancer Research, 61:1207-1213 (2001)). VEGFR-I tyrosine kinase domain is also required for promotion of tumor pathogenesis and metastasis by induction of matrix metalloprotease-9 (MMP-9) in endothelial cells and macrophages (Hiratsuka et al., Cancer Cell, 2:289-300 (2002)). In addition, VEGFR-I has been shown to mediate mobilization and differentiation of P1GF responsive BM-derived precursors (Hattori et al, Nature Medicine, 8:841-849 (2002)). Inhibition of VEGFR-I by an anti-VEGFR-I antibody led to reduction of tumor angiogenesis by preventing recruitment of bone marrow-derived endothelial and monocyte progenitor cells from vascularization in tumors (Lyden et al., Nature Medicine, 7:1194-1201 (2001)). Treatment with an anti-VEGFR-I antibody also effectively inhibited pathological angiogenesis in tumors and ischemic retina in animal models (Lunen et al., Nature Medicine, 8:831-840 (2002)).
This addition to the role of VEGFR-I in angiogenesis, co-expression of VEGF and its receptors is also frequently found in hematological malignant cells and certain solid tumor cells (Bellamy, Cancer Research, 59:728-733 (1999); Ferrer et al., Urology, 54:567-572 (1999); Price et al, Cell Growth Differ., 12:129-135 (2001)). VEGF has been shown to directly induce proliferation, survival, and invasion of VEGF receptor expressing leukemia cells by activation of downstream intracellular signaling pathways through a ligand stimulated autocrine loop (Dias et al, Proc Natl Acad Sd USA, 98:10857-10862 (2001); Gerber et al, J. Mol Med, 81:20-31 (2003)). VEGF stimulation also results in an increased invasiveness of the VEGFR-I expressing breast cancer cells by inducing the activation of ERK1/2 and PI 3/Akt-kinase signaling pathways (Price et al, Cell Growth Differ., 12:129-135 (2001)).
VEGFR-I and its ligands have also been shown to play and important role in inflammatory disorders. VEGF-B deficiency resulted in the reduction of inflammation-associated vessel density and synovial inflammation in models of arthritis (Mould et al, Arthritis Rheum., 48:2660-2669 (2003)). PlGF also plays a critical role in the control of cutaneous inflammation by mediating vascular enlargement, inflammatory cells and monocytes/macrophages, and has been shown to contribute to modulation of atherosclerosis and rheumatoid arthritis in animal models (Luttun et al, Nature Medicine, 8:831-840 (2002); Autiero & Thromb Haemost, 1:1356-1370 (2003)). Treatment with a neutralizing anti-VEGFR-1 antibody suppressed inflammatory joint destruction in arthritis, reduced atherosclerotic plaque growth and vulnerability. The anti-inflammatory effects of the anti-VEGFR-1 antibody were attributable to a reduced mobilization of bone marrow-derived myeloid progenitors into the peripheral blood, a defective activation of myeloid cells, and an impaired differentiation and infiltration of VEGFR-I-expressing leukocytes in inflamed tissues. Thus, VEGFR-I may also be therapeutic target for treatment of inflammation-related disorders.
There remains a need for agents which inhibit VEGF receptor activity, such as fully human monoclonal antibodies (mAbs) specific for VEGFR-I. The anti-VEGFR-1 antibodies may be a useful, novel therapeutic antagonist for treatment of angiogenesis-associated diseases and cancer.